1. Field of the Invention
The present invention relates to a method and circuitry for safely regulating the charge and discharge cycles of implantable grade, rechargeable power sources, utilizing inductively coupled radio frequency energy. Patient safety and power source longevity are vastly improved by the method and circuitry of the system of the present invention. Such safety and longevity are obtained by the steps of: (1) measuring and recording, each charge/discharge cycle, to obtain the corrected capacity of the power source in order to calculate and display, upon interrogation, the remaining operating time of the implanted device, (2) providing within the implanted medical device circuitry for disconnecting the power source upon reaching a pre-selected low voltage in order to prevent deep-discharging the power source below safe limits, (3) providing circuitry for using variable constant current charge rates, (4) providing circuitry for switching to constant voltage to top-off the power source at the completion of the charge cycle, in order to prevent overcharging beyond safe limits, (5) providing within the implanted medical device circuitry for disconnecting the charging circuit from the power source upon the power source reaching a preselected high voltage level, (6) providing circuitry for full-time RF powered operation, in case of failure of the internal power source or for operation of the implanted medical device requiring extremely high power consumption (rather than being powered from the internal power source of the implanted device), (7) providing circuitry for transmitting to a remote receiver, via a telephone link, critical data that can be used by the physician and/or the device manufacturer to assess the performance and condition of the rechargeable power source and the Implantable Medical Device, and (8) providing circuitry for transmitting to the implantable medical device, via a telephone link, new operation parameter value(s).
2. Description of the Prior Art
A number of new, state-of-the-art, implantable medical devices are powered by a rechargeable electrical power source, such as a small volume, large value capacitor (known as a Super-capacitor), or a rechargeable electrochemical cell. These power sources need to be periodically recharged by an external Radio Frequency (RF) Transmitter via inductive coupling in a manner known in the art.
Each type of power source has a different charge and discharge methodology which must be faithfully followed to prevent permanent damage to the power source. In the prior art, the charge/discharge methodology has been factory preset via a specific hardware circuitry, suitable only for the specific power source used to power the implantable device. Furthermore, the prior art circuitry is incapable of properly regulating the charge/discharge cycles of new implantable-grade powersources, such as a Lithium-Ion cell battery.
Heretofore various battery power source charging systems have been proposed. Examples of some of these previously proposed systems are disclosed in the following U.S. patents:
The present invention provides the method, software and hardware to (a) support the correct charge/discharge regimen for different types of power sources, (b) the capability of selecting, via software, the correct regimen of current and voltage limits, and (c) the capability of non-invasively up-grading the regimen, by down-loading, via a direct telemetry link or telephone link, new software revisions incorporating new improvements.
Some new state-of-the-art implantable medical devices are powered by a rechargeable Super-capacitor. One limitation of a capacitive power source is the small amount of charge that it can hold relative to an electrochemical rechargeable cell. In the case of a Super-capacitor powered Implantable Medical Device, when the device requires very high power consumption, its power source must be recharged very frequently. This makes the Super-capacitor impractical as a power source for use in high power consumption implantable medical devices. One obvious solution is to replace the Super-capacitor with an electrochemical cell. However, most implantable-grade, rechargeable electrochemical cells exhibit other critical limitations when used in a hermetically sealed implantable unit. These limitations must be surmounted during the design phase of the charge/discharge regulating circuit for the implanted power source.
One of the power sources most suitable for use in hermetically sealed, rechargeable implantable medical devices, is the Lithium-Ion cell. It offers many advantages, such as relatively high energy density (high capacity), no out-gassing during charge and discharge, high current delivery capabilities and relatively high output voltage. However, it also has some disadvantages, such as some loss of capacity with each recharge cycle (called xe2x80x9cfadexe2x80x9d), and the cell may be permanently damaged if allowed to be deeply discharged or overcharged. The continual loss of capacity (fade), requires the capability of measuring and up-linking (a) the corrected capacity value in mA-hrs, and (b) the power consumption of the Implanted Medical Device, in order to accurately calculate and display the operating time for the Implanted Medical Device. Having the capability of displaying the accurate operating time is extremely helpful to elderly patients for scheduling the next recharge session.
The power management system of the present invention provide a method and circuitry for measuring, on a real-time basis, the current power consumption and elapsed time since the last full charge. This data is used by a microcontroller to calculate (a) the actual capacity (corrected for fade) of the power source, and (b) the xe2x80x9coperating timexe2x80x9d for the Implantable Medical Device. This operating time can be up-linked by the Implantable Medical Device to the RF Transmitter/Charger where it can be displayed to the patient. Thus, the patent is provided, at any time, with an accurate prediction of the operating time as the cell""s capacity slowly fades.
If desired, the work performed by the microcontroller in the power management system/module can be performed by a microcontroller of the Implantable Medical Device. In either event, the following functions are performed:
1. Detecting whether or not an RF sensor line has switched high or low.
2. Controlling the charging rate.
3. Non-invasively changing the charge high voltage limit.
4. Switching to a constant voltage mode to top off the charge on the power source.
5. Non-invasively changing the low voltage limit when the power source is disconnected during discharge.
6. Disconnecting the power source when it reaches the low voltage limit.
7. Reconnecting the power source upon sensing the transmission of RF energy.
8. Disconnecting the power source upon sensing a high temperature.
9. Reconnecting the power source when the temperature drops to a normal level.
10. Measuring the power consumption of the circuitry for the Implantable Medical Device.
11. Measuring the elapsed time since the last full charge.
12. Tracking the actual capacity of the power source.
13. Calculating the operating time left for the Implantable Medical Device.
It is an aspect or objective of the present invention to provide: (1) a method and circuitry for measuring the current drain of the Implantable Medical Device, (2) a method and circuitry for measuring the elapsed time since the last full charge, (3) a method for calculating the actual capacity of the power source (corrected for fade) based on the variable of current drain and the variable of elapsed time, (4) a method for calculating the operating time based on the variable of current drain and the variable of the actual capacity of the power source, (5) a method and circuitry for measuring the voltage of the power source, (6) a method and circuitry to signal the Implantable Medical Device when the power source voltage has reached a certain low value which requires disconnection from the power source, (7) a method and circuitry for disconnecting, during discharging, the power source from the Implanted Medical Device upon the power source reaching a certain low voltage in order to prevent deep discharging of the power source and subsequent damage, (8) circuitry for precisely limiting the charging voltage to the power source in order to prevent overcharging beyond safe limits, (9) a method and circuitry for disconnecting, during charging, the power source from the charging circuit upon the power source reaching a certain high voltage in order to prevent overcharging of the power source and subsequent damage, (10) circuitry for sensing when the electromagnetic waves being transmitted by the RF Transmitter/Charger induce a voltage level above a certain value at the RF Receiver of the Implanted Power Management System, (11) circuitry for reconnecting the power supply inputs of the Implanted Medical Device to the power source upon sensing this induced high voltage level, (12) a method and circuitry for monitoring the temperature of the power source during charging and discharging, (13) circuitry for disconnecting the charging circuitry from the power source if the temperature of the power source raises above a certain level during charging, (14) circuitry for reconnecting the charging circuitry to the power source when the temperature of the power source drops below a certain low value during charging, (15) circuitry for disconnecting the Implanted Medical Device from the power source if the temperature of the power source raises above a certain level during discharging, (16) circuitry for reconnecting the Implantable Medical Device to the power source when the temperature of the power source drops below a certain low value during discharging, (17) a method and circuitry for transmitting to a remote device, via a telephone link, data that can be used by the physician and/or the device manufacturer to assess the performance and condition of the rechargeable power source and the Implantable Medical Device, and (18) a method and circuitry for transmitting via a telephone link to, and setting in, the Implantable Medical Device, new operational parameter value(s).